To examine the effect of UVA irradiation on proteins in skin, an immortal cell line of human keratinocytes (HaCaT-cells) was exposed to UVA and the level of oxidized protein (carbonyl content) was determined. HaCaT is a spontaneously transformed human epithelial cell line from adult skin, which maintains full epidermal differentiation capacity and is not tumorigenic. Exposure of the cells to a single dose of UVA (10 J/cm2) led to an immediate 4- to 5-fold increase in carbonyl content, which was followed by a time-dependent 50% decrease in carbonyl content, caused by proteolytic degradation of the oxidized protein. A similar increase in carbonyl content was obtained by exposure of the HaCaT-cells to a non-toxic concentration (100 microM) of H2O2. In another study, it was demonstrated that the level of oxidized protein in scrapings of Stratum corneum (HSC) from white human subjects increased linearly from 25 to 130 nmol/mg as a function of age over the range of 20 to 80 years. Furthermore, there is an age-dependent increase in the protein carbonyl content of HSC and in the sensitivity of HSC to oxidation by a single dose of UVA. By comparison, the protein in HSC from African-American subjects contained much lower levels of carbonyl groups, at all ages, both before and after exposure to UVA.